1. Field of the Invention
The present invention relates to a method for producing a shape memory alloy (SMA) member having a range of specific physical and mechanical properties and more particularly to the control of the physical and mechanical properties by the introduction of predetermined internal stresses into the alloy prior to a predetermined memory imparting heat treatment.
2. Description of the Prior Art
A nickel-titanium alloy, such as Nitinol (NiTi) is known to have the ability to recover its original shape when deformed in its Martensite and/or Rhombohedral phase(s), and then heated to the Austenite phase. This characteristic of shape memory alloy is generally attributed to the basic chemical composition of the alloy, processing, and the memory imparting heat treatment.
There are a number of articles which describe the aforementioned characteristic of SMA. These include U.S. Pat. Nos. 4,310,354 and 3,174,851 as well as an article from the Naval Surface Weapons Center entitled "Effects of Stresses On The Phase Transformation of Nitinol" (NSWC TR 86-196 1986) and "Effect of Heat Treatment After Cold Working on the Phase Transformation of TiNi Alloy" Transactions of the Japan Institute of Metals, Vol. 28, No. 2 (1987) pages 83-94.
All of these articles are concerned with the generally known processes for making a SMA alloy. This includes the steps of initially selecting an alloy of a predetermined composition, forming the alloy to a desired shape, and subjecting the alloy to a predetermined memory imparting heat treatment. Even though close control of the alloy's chemical composition and memory imparting heat treatment is maintained, a considerable variation in transformation temperatures has been known to occur. This has generally been attributed to process variables and other unknown factors. This limits the use of SMA alloys in applications where more precise transformation temperatures, and other mechanical and physical properties are sought.